Nonclassical Recrystallization

Abstract

Applications in the fields of materials science and nanotechnology increasingly demand monodisperse nanoparticles in size and shape. Up to now, no general purification procedure exists to thoroughly narrow the size and shape distributions of nanoparticles. Here, we show by analytical ultracentrifugation (AUC) as an absolute and quantitative high-resolution method that multiple recrystallizations of nanocrystals to mesocrystals is a very efficient tool to generate nanocrystals with an excellent and so-far unsurpassed size-distribution (PDI_c =1.0001) and shape. Similar to the crystallization of molecular building blocks, nonclassical recrystallization removes "colloidal" impurities (i.e., nanoparticles, which are different in shape and size from the majority) by assembling them into a mesocrystal. In the case of nanocrystals, this assembly can be size- and shape-selective, since mesocrystals show both long-range packing ordering and preferable crystallographic orientation of nanocrystals. Besides the generation of highly monodisperse nanoparticles, these findings provide highly relevant insights into the crystallization of mesocrystals.

Keywords: analytical ultracentrifugation; mesocrystals; nanocrystals; recrystallization; size separation.

Figure 1

Nonclassical recrystallization. First, the “impure” colloidal agglomerate is redispersed in solvent and afterward recrystallized to mesocrystals. The recrystallization step tends to exclude “colloidal impurities”. The crystallization of mesocrystals is reversible. Therefore, “colloidal impurities” can be removed by repeated recrystallization, separating the supernatant from the mesocrystals. This process accumulates nanocrystals of higher quality within the mesocrystals, which can be again redispersed in any solvent.

Figure 2

Important steps of “nonclassical” recrystallization. a–c) TEM images at each stage of the nanocrystals (Batch I) before and after the crystallization of mesocrystals and the supernatant. The nanocrystals from the supernatant were not incorporated in the mesocrystal. d) The normalized diffusion‐corrected sedimentation coefficient distribution narrows after a recrystallization (rc) step. The dotted line presents the polydispersity of the supernatant. e, f) The SEM images clearly demonstrate how the quality of the mesocrystal surface increases after the recrystallization step. g) Representation of a rhombohedral shaped mesocrystal Type 1. Inset: texture‐like WAXS pattern.

Figure 3

Effect of recrystallization on shape selectivity of nanoparticles. a, b, d) 2D plots obtained by 2 DSA‐MC analysis of AUC data (Figure 2 d) showing the relation of sedimentation coefficient and f/f₀ for the nanocrystals (batch I) before and after the crystallization of mesocrystals and supernatant. Insets top right show 3D representations of the plots, the colour gradient indicates the partial concentration of the species. c, e) Exemplary TEM images of the nanoparticle batch after the fifth recrystallization cycle and in supernatant. Orange arrows highlight several particles of non‐cubic shape. The analysis of the shape distribution is shown in Figure S8.

Figure 4

Presentation of other investigated nanocrystal batches and their corresponding mesocrystals from cyclohexane. a–d) Sedimentation coefficient distribution of different nanoparticle batches from cyclohexane before and after a purification step. Batch II–IV contain a majority of larger over smaller “colloidal” impurities, while Batch V contains smaller than larger “colloidal” impurities. Mesocrystals could be obtained for all different nanocrystal batches. Highly monodisperse nanocrystals are received after purification. Please note that the non‐diffusion corrected g(s) envelopes the high‐resolution diffusion‐corrected c(s).

Figure 5

Reversible formation of mesocrystals with different symmetry of the superstructure and the size‐selective recrystallization. SEM images of mesocrystals and their corresponding images after several recrystallization cycles from Batch V. a) After first crystallization in THF b) After the second crystallization in THF c) After the third crystallization in cyclohexane (CH) and d) After the sixth crystallization in THF. e) Normalized c(s) and g(s) before and after three recrystallization cycles.

Figure 6

Evolution of mesocrystals from agglomerates. Recrystallization of colloidal agglomerates and mesocrystals from nanocrystal batch III with initially broader PSD and lower quality using THF and cyclohexane (CH). a–d) SEM images of the agglomerates and mesocrystals obtained after several recrystallization cycles. a) First crystallization, b) second crystallization, c) third crystallization, d) sixth crystallization. e, f) g(s) and cPSD from AUC in toluene before the crystallization and after the third and sixth crystallization.